If you never programmed at all before or if you are interested in starting with the basics, spend the first few months focused on learning Java. Learn the syntax and understand how everything works. You’ll need to be able to create classes, create and call methods, use interfaces as well as know how inheritance works, before you can go to the next step. These are the basics of Java, and you’ll use them extensively when developing Android apps.
Check out the links below first to brush up on Java:
There are plenty of other helpful resources for learning Java:
Programming by doing Java - Great step-by-step guide with challenges along the way
TutsPlus Java Overview - Solid Java overview tutorials
Learn Java Online - Interactive Java Tutorial
CodingBat Java Exercises - Exercises to practice Java
Java tutorial - A programming community & a great place to find the best online programming courses and tutorials.
For a more in-depth look, you can also check out books (available online or in print):
Free Java Book - Solid online book
Best Ways to Learn Java Overview - Overview article
Udemy Free Java Course - Free videos to learn Java
Be sure to check out these particular Java topics as well: